The semiconductor industry has experienced rapid growth due to improvements in the integration density of a variety of electronic components. As semiconductor technologies further evolve, metal oxide semiconductor (MOS) transistors have been widely used in today's integrated circuits. MOS transistors are voltage controlled device. When a control voltage is applied to the gate a MOS transistor and the control voltage is greater than the threshold of the MOS transistor, a conductive channel is established between the drain and the source of the MOS transistor. As a result, a current flows between the drain and the source of the MOS transistor. On the other hand, when the control voltage is less than the threshold of the MOS transistor, the MOS transistor is turned off accordingly.
MOS transistors may include two major categories. One is n-channel MOS transistors; the other is p-channel MOS transistors. According to the structure difference, MOS transistors can be further divided into two sub-categories, planar MOS transistors and vertical MOS transistors. As semiconductor technologies further advance, new power MOS devices have emerged to further improve key performance characteristics such as voltage rating, power handling capability and reliability. For example, lateral double diffused MOS transistors are capable of delivering more current per unit area while maintaining a high breakdown voltage. Lateral double diffused MOS transistors may be alternatively referred to as high voltage MOS transistors.
In order to reduce source, drain and gate resistances of high voltage MOS transistors, a salicide process may be employed to form metal silicide contacts on top of the source, drain and gate electrode regions prior to forming contact plugs connected to the source, drain and gate electrode regions respectively. The most common metal silicide materials are nickel silicide and cobalt silicide. In the salicide process, a thin layer of metal is blanket deposited over the semiconductor substrate. In particular, the thin layer of metal is deposited over the exposed source, drain and gate electrode regions. One or more annealing processes may be applied to the thin layer of metal. These annealing processes cause the metal to selectively react with the exposed silicon of the source, drain and gate electrode regions, thereby forming metal silicide layers on top of the source, drain and gate electrode regions respectively. After the metal silicide layers have been formed, the un-reacted metal is removed. In addition, a plurality of contact plugs may be formed over the source, drain and gate electrode regions.